Milky Way Stars Interact Dynamically

Think about Earth and its population of over 7 Billion people. That’s 7 Billion people who wake up, breathe, live, think, experience, and interact with each other.  The sheer volume of interactions and variation in the human experience is staggering.  Every second you are alive these interactions are happening all around you, and far from you in any corner of the planet.  Millions of people right now feel sad, happy, ecstatic, broken, angry, tired, energetic, and everything in between.

Credit: Illustration by Dana Berry/SkyWorks Digital, Inc.; SDSS collaboration

Now if we go beyond to the Milky Way, where there are more than 50 stars for each and every homo sapiens on the planet, shouldn’t we expect to see an even greater number of interactions and situations? Shouldn’t the sheer statistics mean millions of strange interactions and configurations? The difference is that stars live Billions of years longer than the average human, and they are much further apart, so the vast numbers of interactions do happen more slowly.  But even at the slower rate, with so many stars interacting, we should expect the face of the Milky Way to change quite quickly.  And now, astronomers from New Mexico State University have shown that it does.

Using the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe 100,000 stars during a 4-year period, they were able to determine the origin and history of the stars, including where they formed and when.  This is all possible due to observations of the metallicity of a star, which is like a fingerprint of heavier elements that reside in the atmospheres of stars.

As stars live and die, they fuse lighter elements like hydrogen and helium into heavier elements like oxygen, nitrogen, silicon, and iron.  After a star dies, it seeds the cosmos with these heavier elements, which then help the formation of a new generation of stars, which will have a higher percentage of heavy elements to begin with.  This means that an understanding of metallicity tells us which generation a star belongs to.  The specific ratios of heavy elements can also tell us which stars formed together in the same birth cloud, and gives us the ability to track their movement across space.

Astronomers using APOGEE were able to create a map of relative amounts of 15 heavier elements, and they found that nearly 30% of stars in the study has compositions that indicated they had travelled a significant distance from where they were formed.  Looking at the map in detail, the team realized that the patterns could be explained with a model in which the stars continuously migrated closer to the galactic centre and back out again, carrying them away from their birthplace.  This migration is likely caused by irregularities in the disk of the Milky Way, such as the dense spiral arms.

More data will reveal precisely how the stars have migrated, and a larger population in the study will determine just how often this process occurs.  Either way it seems that the Milky Way has a continuously evolving character, and is a much more dynamic environment than previously thought.


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